Thermionic Cascade Converter Concept for High-Efficiency Space Power

This paper presents trade studies that address the use of the thermionic/AMTEC cell - a cascaded, high-efficiency, static power conversion concept that appears well-suited to space power applications. Both the thermionic and AMTEC power conversion approaches have been shown to be promising candidates for space power. Thermionics offers system compactness via modest efficiency at high heat rejection temperatures, and AMTEC offers high efficiency at modes heat rejection temperature. From a thermal viewpoint, the two are ideally suited for cascaded power conversion: thermionic heat rejection and AMTEC heat source temperatures are essentially the same. In addition to realizing conversion efficiencies potentially as high as 35-40%, such a cascade offers the following perceived benefits:

• Survivability - capable of operation in the Van Allen belts.
• Simplicity - static conversion, no moving parts.
• Long lifetime - no inherent life-limiting mechanisms identified.
• Technology Readiness - Large thermionic database; ATMEC efficiencies of 18% current being demonstrated, with more growth potential available.
• Technology Growth - applicable to both solar thermal and reactor-based nuclear space power systems.

Mechanical approaches and thermal/electric matching criteria for integrating thermionics and AMTEC into a single conversion device are described. Focusing primarily on solar thermal space power applications, parametric trends are presented to show the performance and cost potential that should be achievable with present-day technology in cascaded thermionic/AMTEC systems.